1. Field of the Invention
The invention relates generally to improved electro-optical display devices, and, more particularly, to a display apparatus utilizing a dichroic liquid crystal nematic medium and an electroluminescent backlight to provide enhanced brightness and contrast.
2. Description of the Prior Art
Display devices employing electro-optical materials such as nematic liquid crystal media are well known in the art. Liquid crystal materials are characterized by large changes in optical index of refraction for only modest changes in the electric field at relatively low electric field magnitudes and have other significant advantages, as are widely discussed in the literature. Liquid crystal electro-optic effects are important because they do not require the emission of light; instead, they modify the passage of light through the liquid crystal, either by light scattering, modulation of optical density, or color changes. Further, they provide enhanced readability under high-brightness ambient conditions.
Liquid crystal displays are particularly adapted for use in equipment requiring alphabetical, numeric, or static displays such as used in commercial and military flight deck avionics equipment.
Dichroic liquid crystal displays (LCD) provide excellent reflective (day-time) contrast and readability and obviate the need for polarizing elements. However, under backlighted (night-time) conditions, contrast and readability in this type of display is very poor. Contrast for dichroic liquid crystal displays may be as high as 15:1 for reflective conditions and as low as 5:1 for backlighted conditions. FIG. 1 shows a typical structure of the prior art for providing improved backlighted contrast in dichroic liquid crystal displays. A dual-cell display comprises two superposed substantially identical displays, a front display 10 and a rear display 14. The two displays are energized character for character in parallel. A light source 16, which may be comprised of an array of incandescent bulbs, is utilized as a source of backlighting wherein the backlighting is first passed through the rear display cell 14 and then directed through the front cell 10. This allows the cooperating energized characters of the two cells to transmit a high percentage of the back-lighting, thereby maintaining a bright character. Further, the combined background areas of the superposed cells provides a surface that is nearly opaque and thereby blocks a high percentage of the transmitted backlight. The result is a display which will produce a typical backlighted contrast of 15:1.
Disadvantageously, the dual-cell displays require two independent LCD cells to enhance the contrast. This essentially doubles the complexity of the display, by requiring twice as many cells, functional character segments, and electrical interconnections. The dual-cell display is made further complex by the requirement for precision alignment in an optical sense.
In order to provide satisfactory day-time viewing, the prior art further requires a transflector to provide a reflective surface between the two cells, the reflective surface thereby enhancing readability by reflecting ambient light. This transflector, while it is a good reflector, will typically transmit only ten percent of the available light. Because the backlighting of the dual-cell display must illuminate two independent LCD cells as well as the transflector, the required backlighting must be a very high intensity source. Such a high-intensity backlighting source is physically very large and typically dissipates eighty percent of the total power applied to the display module.
The present invention utilizes a single-cell LCD display, in which the high intensity backlight is replaced by an integral low intensity, low power electroluminescent light source. The integral electroluminescent light source effectively obviates the need for a transflector. Eliminating the requirement for a transflector significantly reduces the total backlighting requirement of the display. The structure of the present invention provides increased backlighted contrast of the display and reduction in physical size, power dissipation, and complexity of the display module.